Alloy



Patented Jan. 2, 1934 PATENT OFFICE ALLOY Howard Scott, Wilkinsburg, Pa., assignor to West-, I inghouse Electric & Manufacturing Company, a corporation of Pennsylvania No Drawing. Application February 8, 1930 Serial No. 427,061

11 Glaims.

My invention relates to alloys and more particularly to alloys having low coeiiicients of expansion at ordinary temperatures.

The principal object of my invention is to provide an alloy containing iron, nickel and cobalt that has a low coefiicient of expansion.

Another object of my invention is to provide proximately 36% nickel has been employed forapplications requiring low expansivity at normal temperatures. This alloy is known in the art as Invar and, under ordinary conditions, has an expansivity of .8 to 2.0 1(l per degree centigrade, although a slightly negative expansivlty can be conferred upon the best grades by cold working. The necessity for cold working, however, restricts its application because heating the alloy counteracts the decreased expansivity which is obtained by the cold-working process.

I have made the discovery that, if cobalt is substituted for part of the nickel in an alloy, such as Invar, in certain definite proportions, an improved product will be obtained having an approximately zero coefficient of expansion at normal temperatures and a high degree of stability although subjected to severe temperature changes. Experiments clearly demonstrate that my improved alloy is superior to Invarin the manufacture of such articles as pendulums, thermostats, gauges, length standards and tape measures.

It is well known that, if nickel is added in certain proportions to iron, the temperature at which the transformation between the gammaand alpha structures of iron takes place 'will be reduced below normal temperatures. When the nickel content is approximately 36%, the transformation point is reduced below that of liquid air, and such alloys, accordingly, have a reversible coefiicient of expansion over a comparatively wide range of temperatures.v An alloy containing about 36% nickel also has the lowest coeflicient of expansion of any known iron-nickel alloy. I have found, howeventhat, if cobalt is substituted for part of the nickel, the coeflicient of expansion of the alloy will be still lower, and the lowexpansion characteristic will be stable over substantially the same range of temperatures.

The substitution of cobalt for nickel, however, raises the temperature at which the transformation of the iron from the gamma to the alpha state takes place, but, if the cobalt which is substituted for they nickel is maintained with- 05 in certain definite proportions, this factor will notadversely aiiect the commercial value of the alloy becausearticles manufactured from it will not ordinarily be subjected to temperatures lower than -100 to 0 centigradle.v

The amount of cobalt that may be substituted for nickel is also dependent upon the amount of carbon and manganese which are present. In my researches, I have found that manganese and carbon are 2.5 and 18 times, respectively, as effective as nickel in reducing the temperature at which the transformation point between garrima and alpha iron takes place, providing the carbon is maintained in solid solution. The amount of carbon which may be maintained in solid solution by the ordinary melting and casting process is limited'to approximately 0.3%, although this amount may be increased to over 0.5% by an appropriate heating operation, such as that of heating the alloy to a temperature of 700 centigrade or higher and then cooling rapidly.

In practicingmy invention, cobalt, iron and nickel, in the proper proportions, are melted together in a suitable furnace, such as an electric furnace. Just before pouring, a small quantity of manganese and silicon are preferably added to render the alloy forgeable, and, in applications requiring a high degree of dimensional stability, the amount of carbon should preferably be maintained below .3%. The alloy may then be cast into ingots in theusual iron or steel molds, after which it may be fabricated in the usual, manner, as by forging, or it may be subjected to a hot or cold rolling operation, and 100 the desiredcarticles formed by the usual punch-v ing or machining processes.

When neither manganese nor carbon is present, the amount of cobalt that may be substituted for part of 'the nickel in an alloy containing a nickel-plus-cobalt content of. 33% to 38% willrange from 2% to 8%. By making such a substitution, the temperature at which the transformation point between gamma and alpha iron takes place will vary between -l00 C. and

- C. The presence of carbon in solid solution in my alloy is beneficial because it does not materially affect the expansion characteristics of the alloy, and, since it is 18 times as effective as nickel in reducing the temperature at which the transformation takes place, it permits the substitution of a greater percentage of 'cobalt for nickel. I have found it highly desirable to have carbon present in amounts ranging up to at least .3% and, for alloys requiring a minimum coeflicient of expansion, the maximum amount of carbon that may be retained in solid solution should be present.

Carbon, in the graphitic form, has practically no effect in reducing the transformation point, so that the amount of cobalt that may be substituted for nickel will depend on the carbon which is in solid solution. I do not wish to limit my invention, however, to alloys containing a low percentage of carbon because when iron-nickelcobalt alloys having a nickel-plus-cobalt content between 33% and 38% are cooled, the carbon either remains in solid solution or separates out in the graphitic form. In certain applications of my improved alloy, it is desirable to'prepare articles of a particular shape by casting. In such instances, the addition of carbon improves the casting properties without materially affecting the expansion properties. When such articles are formed; it 'is desirable, however, to subject the cast article to a heat treatment, to relieve stresses and improve the stability of the alloy, as by heating to approximately 500 C. and then cooling. The amount of carbon that may be added in such cases will depend upon the strength desired in: the finished article. Ordinarily, however, not more than present. I

The addition of manganese is made for the purpose of increasing the forgeability of the alloy. Manganese, however, increases the coefficient of expansion of the alloy, and it is essential that it be maintained as low as possible. For alloys requiring a minimum expansivity, not more than 1% should be employed. It will also be under stood that other elements which will confer forgeability may be. substitutedfor the manga- In an iron-nickeln'zobalt alloy which is substantially free from carbon and manganese and which contains a combined cobalt-nickel content of 36%, 3% of cobalt will raise the temperature of the transformation point to about -100 C. A cobalt content of 8%, in such circumstances, would raise the transformation point to approximately 0 C. In order to produce an alloy of such a type that will have the minimum reversible expansion characteristics at normal temperature, the cobalt content should not exceed 8% and the nickel-plus-cobalt content should not exceed 38%. When carbon and manganese are present, however, the cobalt content may be increased because both carbon and manganese are effective in lowering the temperature at which the transformation point between gamma and 5% should --be.

alpha iron occurs, so that the total amount of cobalt that maybe substituted for the nickel is 2% to 8% plus 3.9 times the percentage of manganese plus 16 times the percentage of carbon,

and the temperature at which the transformation takes place when such a substitution is made, will range between l00 C. and 0 0., depending upon the amount of cobalt substituted for the nickel.

In order that the transformation point of the alloy may be safely maintained below normal temperatures and a minimum expansivity obtained over a widerange of temperatures, Iprefer an alloy containing a combined nickel-and-cobalt content of 36%, of which amou'ht the cobalt will be equal to 3 plus 3.9 times the percentage of manganese plus 16 times the percentage of-car bon. In order to avoid expensive annealing processes, the carbon is preferably maintained at about .3% and the lowest amount of-manganese that is effective in rendering the alloy forgeable is approximately .2%, so that the amount of cobalt in such an alloy will be between 8% and 9%, and the nickel content will be between 26% and 27%. An iron-base alloy .containing 26.8%

nickel, 8.6% cobalt, .3% carbon and .2% mancobalt content of an alloy substantially free from carbon and containing .2% manganese would be 31.6% and 3.8%, respectively. Such an alloy is preferred in articles requiring a high degree of dimensional stability. If the proportion of manganese in such an alloy is relatively high, for instance, .5%, and .3% of carbon is .present, the percentage of the cobalt can be increased to approximately 10%.

While I have described my invention in considerable detail and have given numerous examples, it will be understood that I do not desire to limit myself to the specific details set forth 1 in the foregoing examples, which should be construed as illustrative and not by way of limitation, and, in view of the numerous modifications which may be effected therein without departing from the spirit and scope of my invention, it is 1 desired that onlysuch limitations shall be imposed as are indicated in the appended claims.

I claim as my invention:

1. A low expansion iron-base alloy comprising a combined nickel-and-cobalt content between 33% and 38%, and small amounts up to the order of one percent of additional element from the group consisting of carbon and manganese, the amount of cobalt varying from 2% to 8%, the balance of the alloy being substantially iron 1 and the alloy being of reversible, low expansivity below about 8 l0 expansivity.

2. A low expansion iron-base alloy comprising nickel and cobalt, and small amount of carbon in maximum amount of about 0.5%, the nickelplus-cobalt content varying between 33% and 38%, the amount of cobalt present being equal to from 2% to 8% plus 16 times the amount of carbon that is in solid solution, the balance of the alloy being substantially iron and the alloy being 05. reversible, low expansiviw below about 3x 10- expansivity.

3. A low-expansion iron-base alloy comprising nickel, cobalt, small amounts oicarbon in a maximum of about 0.5% and small amounts of manganese in a maximum of about one percent, the nickel-plus-cobalt content varying from 33 to 33%, the amount of cobalt being equal to 2% to 8% plus 3.9 times the amount of managanese plus 16 times the amount of carbon in solid solution, the balance of the alloy being substantially iron.

42. A low expansion iron-base alloy comprising nickel and cobalt, and small amount of manganese up to 1%, the nickelplus-cobalt content varying between 33% anti 38% and. the cobalt content varying between 2% to 8% plus 3.9 times the manganese content, the balance of the alloy being substantially iron and the alloy being of reversible, low expansivity below about 8x10- expansivity.

5. A low expansion iron-base alloy comprising nickel, cobalt and .2% to 5% manganese, the

nickel-plus-cobalt content varying between 35% and 37% and. the amount of cobalt being equal to 2% to 8% plus 3.9 times the amount of manganese, the balance of the alloy being substantially iron and the alloy being of reversible, low expansivity below about 8 10- expansivity.

6. A low expansion iron-base alloy comprising nickel, cobalt, and small amount of carbon in maximum amount of about 0.5% and the nickelplus-cobalt content varying from 35% to 37% of which amount the cobalt will be equal to 2% to 3% plus 16 times the amount of carbon that is in solid solution, the balance of the alloy being substantially iron and the alloy being of reversible, low expansivity below about 3 10- expansivity.

7. A low expansion iron-base alloy comprising nickel, cobalt with carbon in maximum amount of about 0.5%, 2% to .5% manganese, the nickelplus-cobalt content varying from 35% to 37%, and the cobalt content being equal to 2% to 8% plus 3.9 times the amount of 1". a. 1n anese plus 16 times the amount oi carbon that is in solid solution, the balance 0! the alloy being substantially iron and the alloy being of reversible, low expansivity below about 8 l0- expansivity.

8. A low-expansion iron-base alloy containing approximately 25 to 32% nickel, 10% to 2% cobalt in accordance with the amount of nickel present, and small amounts of manganese and carbon in maximum amounts of about 1% and 0.5% respectively, the balance of the alloy being substantially iron.

9. A low-expansion iron-base alloy containing approximately 25 to 32% nickel, 10% to 2% cobalt and small amounts of manganese and carbon in maximum amounts of about 0.2% and 0.3% respectively, the balance of the alloy being substantially iron.

10. A low expansion article of manuiacture comprising a ferrous-base alloy containing nickel and cobalt, the cobalt being about eight percent and the nickel-plus-cobalt content of said alloy amounting to 33% to 38% the balance of the alloy being substantially iron and the alloy being of reversible, low expansivity below about 8x l0- expansivity.

11. A low expansion iron-base alloy contain ing nickel, cobalt and small amounts up to the order of one percent of additional element from the'group consisting of silicon, aluminum, magnesium, manganese and carbon, the nickel-pluscobalt content varying between 33% and 38% and the amount of cobalt ranging from about 2% to10%, the balance of the alloy being substantially iron, and the alloy being of reversible,

low expansivity below about B 10 expansivity. l

1 HOWARD soo'rr.

no I 

